One type of generator assembly is a so-called variable reluctance generator wherein a magnetic circuit powered by a permanent magnet and passing through a coil is periodically opened and closed. The AC voltage induced in the coil as a result of the oscillating magnetic field may be used to generate energy or to drive other devices.
In some applications, the room for a stator encompassing the rotor may be limited and be available only on one side. For example in train bogies with saddle type suspension, it may be possible to arrange additional structures on or in a saddle adapter, whereas structures arranged below the axle may be excluded for security reasons and extreme environmental conditions.